Isolation and Characterization of Flavonol Converting Enzymes from Mentha piperita Plants and from Mentha arvensis Cell Suspension Cultures

Abstract

Peroxidases from several plants, including horseradish peroxidase, were capable of converting flavonols to the corresponding 2,3-dihydroxyflavanones in presence of H₂O₂ . Contrastingly, protein extracts from Mentha piperita plants and Mentha arvensis cell suspension cultures perform ed the same enzymatic step in absence of H₂O₂ , but only with quercetin, not with kaempferol. H₂O₂-independent, quercetin converting enzymes were isolated and purified from these extracts, and they could be classified in two groups according to the extent of stimulation of the enzyme reaction by H₂O₂ . Enzymes from group I were stimulated by exogenous H₂O₂ , and they resembled horse­ radish peroxidase in several aspects. They possessed IAA oxidase activity, but quercetin was the preferred substrate. Enzymes from group II from the plants appeared to be a distinctly different set of enzymes. They were not stimulated by H₂O₂ , but required molecular oxygen and converted only 3,3′,4′-trihydroxyflavones under aerobic conditions. Also, they showed no Soret-bands and possessed no IAA oxidase activity. These proteins appear to be a new class of enzymes participating in the first step of flavonol degradation in plants.